Grinding

After coffee beans have been ground into powder, rapid oxidisation results from the increased surface exposure to air, and the oils contained in coffee beans generate acidity from oxidization. The method of brewing is determined by the fineness of coffee grounds. Where the brewing time of the machine is shorter, the surface of the grounds must expand for increased contact with water; conversely, if the brewing time is longer, the grounds must be coarser.

Simply put, the longer the extraction time, the coarser the grind should be to avoid over-extraction that brings out the bitter taste of coffee; the shorter the extraction time, the finer the grind should be to avoid under-extraction. The finer the grind, the more bitter the taste is; the coarser the grind, the less bitter the taste is.